Bonding of thermoset composite structures

ABSTRACT

A pair of thermoset composite structures (19,19&#39;) are bonded together by a pair of adhesive strips (10,10&#39;). An adhesive strip (10) is comprised of a layer of semi-crystalline thermoplastic material (12), a layer of amorphous thermoplastic material (14), a layer of dry fiber reinforcement (16) partially embedded in the layer of amorphous thermoplastic material (14), and a layer of thermosetting resin (18) covering the exposed fibers of the dry fiber reinforcement. An adhesive strip is bonded to the bonding surface (20) of the structure (19) during the curing process for the thermoset composite structure. A resistance heating element is placed between the bonding surfaces (20,20&#39;), the bonding surfaces (20,20&#39;) are pressed together, and electrical energy is passed through the heating element (24) to heat the joint and fuse the thermoplastic adhesive layers together. The dry fiber reinforcement (16) forms a mechanical lock between the cumulative layer of thermoplastic adhesive (32) and the thermosetting structures (19,19&#39;) to provide the bond strength.

This application is a continuation of 07/628,245 filed Dec. 17, 1990.

TECHNICAL FIELD

This invention relates to a method for bonding thermoset compositestructures, and more particularly to the utilization of a thermoplasticadhesive layer to bond a pair of thermoset composite structurestogether.

BACKGROUND ART

Thermoset composites have received widespread use in the aircraftindustry as a structural material. This type of composite can provide alight weight structure of relatively high strength. To obtain a highstrength structural bond with thermoset composites currently requirescostly methods of bonding combined with mechanical fasteners.

One method of bonding thermoset composite structures together is toplace thermoset resin film adhesive material between the surfaces to bebonded, followed by applying heat and pressure to the joint zone. Thisprocess is not very practical as it involves the application ofrelatively high heat (250°-350° F.) and pressure (25 psi. minimum) forextended periods of time (2 hours minimum at temperature). Thermalexpansion can become a problem when large areas of structure are heatedas required for this process. This is due to differing rates of thermalexpansion of localized structure which causes thermal strain when thestructure is cooled and can cause deformations. Another drawback is thatthermoset adhesives can only be processed once and the bond is notreversible for repair or replacement of subsequently damagedsub-structure.

A second method currently used involves attaching together thermosetcomposite structure using mechanical fasteners combined with roomtemperature curing thermoset paste adhesive applied to joint surfaces.This method requires that the components being joined be pre-fitted anddrilled for pilot holes, disassembled, cleaned and bond prepared.Adhesive is then applied to joint surfaces, the components are thenreassembled, and the adhesive cured (usually 24 hours at roomtemperature). The next step is to drill and ream holes for each fastenerand finally the fasteners are installed. This method is costly due tothe use of mechanical fasteners and the labor intensive nature of themethod.

An alternative to these processes is to use a film of thermoplasticadhesive between the thermoset composite structures being joinedtogether. Since thermoplastic may be repeatedly melted and refused, thebonding process can be reversed as needed. Unfortunately, currentthermoset resins will not chemically adhere with sufficient strength tosuitable thermoplastic adhesive materials to meet aircraft requirementsfor joining structural components. Therefore this method is notpractical in situations requiring high strength bonds.

DISCLOSURE OF INVENTION

An object of the invention is an improved method of joining togetherthermoset composite structures.

Another object of the invention is a method of bonding thermosetcomposite structures which permits reversing of the bonding process toallow repair or replacement of sub-structure.

According to the present invention, thermoset composite structures arebonded together using thermoplastic adhesive strips which are comprisedof a layer of suitable thermoplastic material, a ply of dry fiberreinforcement which is partially embedded in the layer of thermoplasticmaterial, and the exposed dry fiber reinforcement is coated with athermoset resin. The surfaces of uncured thermoset composite structuresare prepared by positioning adhesive strips with the thermoset resinside towards the composite structures. The composite structures are thencured by conventional means. Subsequent to curing, the bonding surfaces(now coated with the thermoplastic adhesive strip) are pressed togetherand heated to fuse the joint. During this process sufficient heat isapplied locally to melt and fuse the thermoplastic adhesive materialwithout degrading the adjacent composite structure. The joint is allowedto cool using the composite structure as a heat sink.

The layer of dry fiber reinforcement, which bonds to both thethermoplastic adhesive material and the thermoset material (during thecure cycle), provides the means to mechanically lock the thermosetcomposite structure to the thermoplastic adhesive. This eliminates theneed for a chemical bond to join the two components. The use of athermoplastic material as an adhesive allows the bonding process to beeasily and quickly reversed for repair or replacement of the thermosetstructure.

Further, the heat applying means is a heating strip comprised of aresistance heating element embedded within two layers of thermoplasticadhesive material. The heating strip is placed between the bondingsurfaces of the cured structures, pressure is applied to the joint andelectrical energy, sufficient to heat the joint to above the meltingtemperature of the thermoplastic material, is passed through the heatingstrip for a duration sufficient to fuse the layers of thermoplasticadhesive material. The heating element remains within the joint afterbonding and provides a convenient mechanism to reheat the joint andreverse the bonding process.

Although the invention described is particularly useful for bondingtogether thermoset composite structures on aircraft, it should beunderstood that the invention is equally well suited to any otherapplication in which thermoset composite components are joined togetherin both structural and non-structural applications.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of the exemplary embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a thermoplastic adhesive strip.

FIG. 2 is a sectional view of a woven thermoplastic adhesive strip.

FIG. 3 is a sectional view of a heating strip.

FIG. 4 is a sectional view of a pair of bonding surfaces prepared forbonding.

FIG. 5 is a perspective view of a pair of thermoset composite structuresafter bonding, partially cut away to show the bonding layers.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, a thermoplastic adhesive strip 10 is comprisedof a layer of semi-crystalline thermoplastic material 12, a layer ofamorphous thermoplastic material 14, a layer of dry fiber reinforcement16, and a layer of thermoset resin 18. The size of the strip 10 isdependant on the size of the structures to be bonded together. Forillustrative purposes the thickness of the layer of semi-crystallinethermoplastic material is 0.004"; the thickness of the amorphousthermoplastic material is 0.003"; the thickness of the dry fiberreinforcement is 0.006; and the thickness of the layer of thermosetresin is sufficient to coat the exposed layer of dry fiberreinforcement.

The fabrication of the adhesive strip involves several steps. First, thelayers of semi-crystalline thermoplastic material 12 and the amorphousthermoplastic material 14 are initially fused together by raising thetemperature of the two layers 12,14 above the melting point of thesemi-crystalline thermoplastic material and pressing the layers of 12,14together under approximately 5 to 20 psi. Since semi-crystallinethermoplastic material has a higher melting temperature than amorphousthermoplastic material, the two layers 12,14 become intimately fused.

A typical semi-crystalline thermoplastic material ispolyetheretherketone (PEEK), which has a melting temperature ofapproximately 650° F., and a typical amorphous thermoplastic material ispolyetherimide (PEI), which has a melting temperature of approximately480° F. PEEK and PEI will be utilized in this description forillustrative purposes.

Second, the layer of dry fiber reinforcement 16 is embedded into thelayer of PEI 14 by laying up the dry fiber reinforcement 16 onto thelayer of PEI 14, applying pressure of approximately 5 to 15 psi, andheating the strip to a temperature above the melting temperature of PEI,but less than the melting temperature of PEEK. Since a temperature lessthan the melting temperature of PEEK is used, the layer of PEEK 12 actsas a barrier and prevents the layer of dry fiber reinforcement 16 frominserting beyond the PEI layer 14. In this way a portion of the dryfiber reinforcement 16, which is thicker than the layer of PEI 14, isbonded with the layer of PEI 14, and the remainder is exposed (i.e. notembedded). The last step is to coat (`wet-out`) the exposed dry fiberreinforcement 16 with a thermoset resin 18.

An alternative method for fabricating a thermoplastic adhesive strip isshown in FIG. 2. A woven thermoplastic adhesive strip 48 is comprised ofdry fiber reinforcement strands 50 woven together with commingledstrands 52, which are formed by intimately fusing dry fiberreinforcement strands and spun thermoplastic filaments. The weavingproduces a material which is predominantly dry fiber reinforcement onone side and commingled strands on the other side. The woven material ispressed onto a layer 54 of thermoplastic material, which is thinner thanthe woven material, with the side which is predominantly commingledstrands adjacent to the layer of thermoplastic material, and sufficientheat is supplied to melt the layer of thermoplastic material. Aftercooling, the dry fiber reinforcement strands are fused with andpartially embedded in the layer of thermoplastic material. The exposeddry fiber strands are then coated with thermoset resin.

Another alternative (not shown) is to weave dry fiber reinforcementstrands with thermoplastic filaments so as to produce a woven materialwith predominantly dry fiber reinforcement strands on one side and acombination of dry fiber reinforcement strands and thermoplasticfilaments on the other side. As before, the woven material is pressedonto a layer of thermoplastic material which is thinner than the wovenmaterial, with the woven material which is a combination of dry fiberand thermoplastic filaments adjacent to the layer of thermoplasticmaterial, and sufficient heat is supplied to melt the thermoplasticmaterial. After cooling, the dry fiber reinforcement strands are fusedwith and partially embedded in the layer of thermoplastic material. Theexposed dry fiber strands are then coated with thermoset resin.

An adhesive strip 10 is placed on an uncured thermoset compositestructure 19. The adhesive strip is placed so that the layer ofthermoset resin coated, dry fiber reinforcement 16 is adjacent to thecomposite structure 19. The structure 19, with adhesive strip 10 inplace, is cured as normally required for the thermoset composite toproduce a bonding surface 20. During cure the thermoset resin 18 willwet-out (impregnate) the exposed dry fiber reinforcement 16, which thenbecomes anchored to the thermoset structure 19. In this way the dryfiber reinforcement 16 forms a mechanical lock between the thermosetstructure 19 and the thermoplastic adhesive strip 10, as shown in FIG.4.

Referring now to FIG. 3, a heating element 22 is comprised of aresistance heater 24 embedded between two layers 26,28 of thermoplasticadhesive, in this case PEEK. The resistance heater 24 is assembled witha pair of electrical leads 25,27, which are electrically connected tothe resistance heater 24, and at least one thermocouple 29 fortemperature sensing. The heating element 22 is fabricated by fusing theresistance heater 24, assembled with leads 25,27 and thermocouple 29,between the two layers 26,28 of PEEK. The resistance heater 24 as shownis a thin metallic sheet, but may be fabricated from any conductivematerial and need not be in the form of a sheet. Other forms ofelectrical heating element materials which may be used are: etchedfoils, pierced and stretched foils, stamped elements, or wound wireserpentine.

A pair of cured structures 19,19' are bonded together by placing theheating element 22 between the prepared bonding surfaces 20,20' with thebonded adhesive strips 10,10', as shown in FIG. 4. Pressure ofapproximately 10 to 30 psi is applied to the joint and electrical energyis passed through the heating element by means of an electrical powersupply 30. The joint is heated above the melting temperature of PEEK.This temperature is maintained for a sufficient time to fuse thethermoplastic adhesive in the joint.

For the adhesive strips as described above it is believed that thetemperature of the joint should be raised above the melting temperatureof PEEK (650° F.) in approximately one minute and maintained at thattemperature for approximately 15 seconds. The rate of temperatureincrease and duration of the heating process is limited by the heattransfer rate of the joint and structure in order to avoid degradationof the adjacent thermoset composite structures 19,19'. Upon terminationof the electrical energy supply, the joint is allowed to cool by usingthe bonded structures 19, 19' as heat sinks. The entire fusion cycleshould take approximately two to three minutes.

In certain instances the availability of local structure to act as aheat sink may vary along the length of the joint. In these instancesheat balancing may be required in order to ensure uniform heating andcooling of the joint during the bonding process.

After the cooling period is completed, the structures are bonded asshown in FIG. 5. The various layers of thermoplastic adhesive 32 arefused together and the dry fiber reinforcement 16 is embedded andencapsulated by the thermoplastic adhesive 32 and the thermoset resin ofthe thermoset composite structures 19,19'. The dry fiber reinforcement16 provides a mechanical lock, between the thermoplastic adhesive 32 andthe thermoset composite structures 19,19', which provides the bondstrength. The heating element 24 remains in place and the electricalleads 25,27 remain attached to the heating element to allow for laterreheating of the joint to permit repair or replacement of the bondedstructures 19,19'.

The replacement procedure for a bonded structure would comprise thefollowing steps. First, the joint would be heated by supplyingelectrical energy to the heating element 22 sufficient to heat the jointabove the melting temperature of the thermoplastic adhesive. The processwould be maintained until the thermoplastic adhesive was melted, thejoint could be separated, and the damaged structure could be removed.Second, the replacement structure, which would consist of a curedthermoset composite structure surfaced with thermoplastic adhesive inthe joint zone (similar to the initial structure as shown in FIG. 4), ispositioned in place of the removed structure. Third, a new heatingelement, which is positioned in the joint, is heated by supplyingelectrical energy as discussed previously for bonding structures. Theentire replacement process, assuming a cured structure and new heatingelement are readily available, may be completed in a matter of minutes.

Although certain thermoplastic materials (PEEK and PEI) are suggested asparticularly useful for the specific embodiment illustrated in FIGS. 1to 5, the selection of these materials is not limiting and it should beunderstood that other thermoplastic materials may be equally applicableto the present invention. In, addition the present invention may, ifdesired, be practiced by using a single type of thermoplastic materialas long as a layer of partially exposed dry fiber reinforcement (notembedded in the thermoplastic material) is maintained for embedding inthe thermoset composite structure during the lay-up and curing process.

Although the invention has been shown and described with respect toexemplary embodiments thereof, it should be understood by those skilledin the art that various changes, omissions and additions may be madetherein and thereto, without departing from the spirit and the scope ofthe invention.

What is claimed is:
 1. A method for bonding a thermoplastic adhesivestrip to an uncured thermoset composite member to provide a curedthermoset composite member having a thermoplastic bonding surface,comprising the steps of:(A) preparing said thermoplastic adhesive strip,said preparation step including the substeps of(A1) partially embeddinga layer of dry fiber reinforcement into a layer of thermoplasticmaterial so that a portion of said dry fiber reinforcement layer isexposed, and (A2) applying thermoset resin to coat said exposed portionof said partially embedded dry fiber reinforcement layer to form saidthermoplastic adhesive strip; (B) laying up said thermoplastic adhesivestrip on the uncured thermoset composite member so that saidresin-coated exposed portion of said thermoplastic adhesive strip abutsthe uncured thermoset composite member; and (C) curing said layed-upthermoplastic adhesive strip to the uncured thermoset composite memberto provide the cured thermoset composite component having saidthermoplastic adhesive strip anchored thereto, said thermoplasticmaterial layer of said anchored thermoplastic adhesive strip definingsaid thermoplastic bonding surface of the cured thermoset compositemember.
 2. The method of claim 1 for bonding a thermoplastic adhesivestrip to a thermoset composite member wherein said partially embeddingstep includes the substeps of:intimately fusing together a layer ofsemi-crystalline thermoplastic material and a layer of amorphousthermoplastic material to form said layer of said thermoplasticmaterial, (ii) pressing said layer of dry fiber reinforcement onto saidlayer of amorphous thermoplastic material, the thickness of said layerof dry fiber reinforcement being greater than the thickness of saidamorphous thermoplastic material layer, (iii) applying heat to melt saidlayer of amorphous thermoplastic material, but not to melt saidsemi-crystalline thermoplastic material layer so that said dry fiberreinforcement layer is partially embedded in said amorphousthermoplastic material layer wherein a portion of said pressed dry fiberreinforcement layer is exposed, and (iv) cooling said layer ofthermoplastic material having said dry fiber reinforcement layerpartially embedded therein.
 3. The method according to claim 1 whereinsaid partially embedding step comprises the substeps of:(i) intimatelyfusing dry fiber reinforcement strands with spun thermoplastic filamentsto form commingled strands, (ii) weaving said commingled strands withdry fiber reinforcement strands to form a woven material havingpredominantly said dry fiber reinforcement strands on one side and saidcommingled strands on the other side, said dry fiber reinforcement sideof said woven material defining said layer of dry fiber reinforcement,(iii) pressing said woven material onto said layer of thermoplasticmaterial so that said commingled strands are adjacent said layer ofthermoplastic material, (iv) applying heat to melt said thermoplasticmaterial layer to partially embed said woven material therein wherein aportion of said dry fiber reinforcement strands are exposed, and (v)cooling said layer of thermoplastic material having said woven materialpartially embedded therein.
 4. The method according to claim 1 whereinsaid partially embedding step comprises the substeps of:(i) weavingstrands of dry fiber reinforcement and strands of spun thermoplasticfilaments to form a woven material having predominantly said dry fiberreinforcement strands on one side and a combination of said dry fiberreinforcement strands and said thermoplastic filament strands on theother side, said dry fiber reinforcement side of said woven materialdefining said layer of dry fiber reinforcement, (ii) pressing said wovenmaterial onto said layer of thermoplastic material so that saidcombination of dry fiber reinforcement strands and said thermoplasticfilament strands are adjacent said layer of thermoplastic material,(iii) applying heat to melt said thermoplastic material layer topartially embed said woven material therein wherein a portion of saiddry fiber reinforcement strands are exposed, and (iv) cooling said layerof thermoplastic material having said woven material partially embeddedtherein.
 5. A method for reversibly bonding thermoset compositestructures together, comprising the steps of:(A) providing a bondingsurface on each of first and second uncured thermoset compositestructures to be reversibly bonded together, said bonding surfaceproviding step including the substeps of(A1) partially embedding a layerof dry fiber reinforcement into a layer of thermoplastic material sothat a portion of said dry fiber reinforcement layer is exposed, (A2)applying thermoset resin to coat said exposed portion of said partiallyembedded dry fiber reinforcement layer to form a thermoplastic adhesivestrip, (A3) laying up said thermoplastic adhesive strip on each thefirst and second uncured thermoset composite structures, respectively,so that said resin-coated exposed portion of each said thermoplasticadhesive strip abuts the respective first and second uncured thermosetcomposite structures, and (A4) curing each said layed-up thermoplasticadhesive strip to the respective first and second uncured thermosetcomposite structures to provide first and second cured thermosetcomposite structures each having said thermoplastic adhesive stripanchored thereto, said thermoplastic material layer of said anchoredthermoplastic adhesive strips defining said bonding surface for thefirst and second cured thermoset composite structures, respectively; (B)pressing said bonding surfaces of the first and second cured thermosetcomposite structures together wherein said abutting bonding surfacesdefine a bonding joint between the first and second cured thermosetcomposite structures; (C) applying heat to said bonding joint to fusesaid thermoplastic material layers of said thermoplastic adhesive stripsof the first and second cured thermoset composite structures to form abond layer of thermoplastic material therebetween; and (D) cooling saidbond layer of thermoplastic material wherein the first and secondthermoset composite structures are reversibly bonded together; andwherein said bond layer between the first and second thermoset compositestructures is reversible by heating said bonding joint to melt said bondlayer so that the first and second cured thermoset composite structuresare separable with said respective thermoplastic adhesive stripsanchored thereto.
 6. The method according to claim 5 wherein saidpartially embedding step comprises the substeps of:(i) intimately fusingtogether a layer of semi-crystalline thermoplastic material and a layerof amorphous thermoplastic material to form said layer of thermoplasticmaterial, (ii) pressing said layer of dry fiber reinforcement onto saidlayer of amorphous thermoplastic material, the thickness of said layerof dry fiber reinforcement being greater than the thickness of saidamorphous thermoplastic material layer, (iii) applying heat to meltlayer of amorphous thermoplastic material, but not melt semi-crystallinethermoplastic material layer so that said dry fiber reinforcement layeris partially embedded in said amorphous thermoplastic material layerwherein a portion of said pressed dry fiber reinforcement layer isexposed, and (iv) cooling said layer of thermoplastic material havingsaid dry fiber reinforcement layer partially embedded therein.
 7. Themethod according to claim 5 wherein said partially embedding stepcomprises the substeps of:(i) intimately fusing dry fiber reinforcementstrands with spun thermoplastic filaments to form commingled strands,(ii) weaving said commingled strands with dry fiber reinforcementstrands to form a woven material having predominantly said dry fiberreinforcement strands on one side and said commingled strands on theother side, said dry fiber reinforcement side of said woven materialdefining said layer of dry fiber reinforcement, (iii) pressing saidwoven material onto said layer of thermoplastic material so that saidcommingled strands are adjacent said layer of thermoplastic material,(iv) applying heat to melt said thermoplastic material layer topartially embed said woven material therein wherein a portion of saiddry fiber reinforcement strands are exposed, and (v) cooling said layerof thermoplastic material having said woven material partially embeddedtherein.
 8. The method according to claim 5 wherein said partiallyembedding step comprises the substeps of:(i) weaving strands of dryfiber reinforcement and strands of spun thermoplastic filaments to forma woven material having predominantly said dry fiber reinforcementstrands on one side and a combination of said dry fiber reinforcementstrands and said thermoplastic filament strands on the other side, saiddry fiber reinforcement side of said woven material defining said layerof dry fiber reinforcement, (ii) pressing said woven material onto saidlayer of thermoplastic material so that said combination of dry fiberreinforcement strands and said thermoplastic filament strands areadjacent said layer of thermoplastic material, (iii) applying heat tomelt said thermoplastic material layer to partially embed said wovenmaterial therein wherein a portion of said dry fiber reinforcementstrands are exposed, and (iv) cooling said layer of thermoplasticmaterial having said woven material partially embedded therein.
 9. Themethod of claim 5 wherein said heating step (C) further comprises thesubsteps of:placing a heating element between said bonding surfacesprior to pressing said bonding surfaces together; and passing electricalenergy through said heating element to heat said bonding joint above themelting temperature of said thermoplastic material layers of saidthermoplastic adhesive strips to fuse said thermoplastic material layersof said thermoplastic adhesive strips of the first and second curedcomposite structures to form said bond layer of thermoplastic materialtherebetween.